In the past, a technique of image correlation by means of a lens has been used in which a two-dimensional Fourier transform of the image of an object is used to illuminate a hologram of the Fourier transform of a reference, and retransforming the result with another lens to get the correlation between the object and the reference is taught by A. Vander Lugt and this device is known as an optical matched filter. This technique is probably the mathematical optimum way of comparing an object to a reference. It has the special property of spatial invariance, and correlation occurs regardless of where the object is in the field of view. The location of the spot of light in the correlation plane describes the location of the object. This technique is robust against many forms of object clutter and provides multiple independent correlation spots if there are multiple images in the field of view. This matched filter technique has the disadvantage that it does not compensate for changes of scale or rotation of the object. Typically, a ten percent change of these parameters is enough to destroy the correlation. Another technique, known as Fourier plane detection, is taught by Nicholas George. The image of the object is Fourier transformed as above, but a detector array, rather than a holographic matched filter, is placed in the Fourier plane. George teaches a specially shaped detector, composed of annular ring elements in one half of the detector plane, and wedge elements in the other half. This detector is illustrated in FIG. 1 of the drawing and is disclosed in U.S. Pat. No. 3,689,772, which issued on Sept. 5, 1972.This detector is especially suited to Fourier plane detection, as the center symmetry means that the same element of information will fall on a ring or on a wedge structure of the detector. The ring structure then indicates the spatial frequency of a feature of information and the wedge structure indicates the angular direction of that feature. The ring, wedge information is placed into a digital computer, and any of a variety of algorithms are used to attempt to classify (recognize) the object being detected.
This Fourier plane detection technique has the disadvantage that it does not determine object location. It determines if a desired object is in the field of view, but cannot determine where it is in the field of view. It has the disadvantage that multiple images in the field of view will not necessarily result in multiple determinations. It also has the disadvantage that unwanted information in the field of view such as cutter is superimposed on the desired information so that the system is not highly robust to clutter.
Accordingly, it is an object of this invention to provide a hybrid optical correlator which uses a Fourier plane detector to adjust the image of an object so that a matched filter can provide a correlation, independent of scale and orientation of the object.
Another object of this invention is to provide a hybrid optical correlator which is a hybrid of the matched filter technique and the Fourier plane detection technique.
Another object of this invention is to provide a hybrid optical correlator which takes advantage of the strengths of both a matched filter correlation technique and the Fourier plane detection technique and uses these strengths to counterbalance the weaknesses of each of the other techniques.
Other objects and advantages of this invention will be obvious to those skilled in this art.